Area Forecast Discussion
Issued by NWS Riverton, WY

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000
FXUS65 KRIW 252018
AFDRIW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
118 PM MST TUE NOV 25 2014

.SHORT TERM...TODAY THROUGH THURSDAY NIGHT

UPGRADED WINTER WEATHER ADVISORIES TO WINTER STORM WARNINGS ACROSS
THE MOUNTAINS AND WESTERN VALLEYS. ISSUED WINTER STORM WARING FOR
BIGHORN COUNTY WITH WINTER WEATHER ADVISORIES FOR THE REST OF THE
BIGHORN BASIN AND JOHNSON COUNTY. ALSO ISSUED WINTER WEATHER ADVISORY
FOR THE UPPER GREEN RIVER BASIN. REPORTS OF 6 TO 7 INCHES IN STAR
VALLEY AND 4 TO 6 INCHES IN BIGHORN COUNTY TODAY.

ISSUED HIGH WIND WARNING FOR UPPER WIND RIVER CANYON...SOUTHERN
LINCOLN...AND SWEETWATER COUNTIES FROM 11 AM MST THROUGH 11 PM MST THIS
EVENING FOR 35 TO 45 MPH WINDS GUSTING TO 60 MPH.

INCREASED SNOWFALL AND CHANCE OF PRECIPITATION ACROSS THE
MOUNTAINS AND NORTH. SNOWFALL OF 1 TO 3 INCHES IN THE LOWER
ELEVATIONS OF THE NORTH. COLD FRONT COMING SOUTH AND WINDS HAVE
DECREASED ACROSS THE NORTH.

UPDATE... UPDATE TO ADD THE STAR VALLEY INTO THE WINTER WEATHER
ADVISORY MIX THROUGH 00Z. SNOW COMING DOWN PRETTY HARD RIGHT NOW
AND WITH TEMPERATURES ONLY IN THE LOWER TO MID 20S IT WILL REMAIN
SNOW FOR QUITE A WHILE. COULD TRY AND CHANGE OVER LATER THIS
AFTERNOON ONWARD BUT THEY ARE UNDER A HIGH RISK FOR THE NEXT 6-9
HOURS OF MORE MODERATE TO HEAVY SNOW. WILL ALSO HAVE TO WATCH THE
LOWER ELEVATIONS UP TO THE JACKSON VALLEY FOR POTENTIAL UPGRADE IF
HEAVIER SNOW PERSISTS AS SNOWFALL RATES AROUND AN INCH PER HOUR
ARE QUITE POSSIBLE FROM NOW THROUGH AT LEAST THE MORNING.

ONCE AGAIN WE THE ACTIVE PATTERN WE WILL DIVIDE THE FORECAST UP
BETWEEN WEST AND EAST OF THE DIVIDE.

WEST OF THE DIVIDE...THE POCATELLO RADAR IS FILLING RATHER NICELY AS
THE NEXT SYSTEM MOVES IN FROM THE WEST. WARM ADVECTION SNOW IS NOW
SPREADING INTO THE WESTERN MOUNTAINS. WE STILL ANTICIPATE THE THEN
HEAVIEST SNOW TO BE FROM MID MORNING INTO THE AFTERNOON WITH THE
BEST MOISTURE AND WARM ADVECTION. FOR THE MOST PART...THE CURRENT
HIGHLIGHTS LOOK PRETTY DARN GOOD AND WE WILL LET THEM RIDE FOR
NOW. THERE IS ONE EXCEPTION HOWEVER...THAT IS THE JACKSON VALLEY.
THIS ONE IS A TOUGH CALL. THE PROBLEM IS THE DIFFERENT IN THE
MODELS AND THE 700 MILLIBAR TEMPERATURES THAT WILL BE CRITICAL.
THERE IS A DIFFERENCE WITH THE MODELS THOUGH. THE GFS IS WARMER
WITH 700 MILLIBAR TEMPERATURES RISING TO AROUND -3 TO -3.5 WHICH
WOULD PUSH THE SNOW LEVELS UP TO AROUND 7000 FEET AND BRING SOME
MIXING AND POSSIBLY CHANGING OVER TO RAIN. THE NAM MEANWHILE IS
COLDER WITH TEMPERATURES AROUND MINUS 4 TO 5 WHICH WOULD KEEP MOST
OF THE VALLEY AS ALL SNOW. FOR NOW WE DECIDED TO COMPROMISE...WE
WILL ISSUE A WINTER WEATHER ADVISORY. HOWEVER...THE BEST CHANCE OF
ADVISORY LEVEL SNOW WOULD BE NORTH OF THE JACKSON HOLE AIRPORT
TOWARD MOOSE AND MORAN JUNCTION. WE WILL SPECIFY THAT IN THE
ADVISORY. SNOW WILL BEGIN TO LIGHTEN TONIGHT AND ESPECIALLY
TOMORROW AS RIDGING BEGINS TO BUILD ACROSS THE AREA FROM THE SOUTH
AND WEST AND THE BEST UPPER LEVEL SUPPORT LIFTS OUT. COVERAGE WILL
CONTINUE TO DECREASE THROUGH THE DAY WEDNESDAY BEFORE ENDING FOR
THE MOST PART BY WEDNESDAY EVENING. AT THIS TIME...WE HAVE DRIED
OUT THE WEATHER FOR THURSDAY FOR ALL AREAS WITH THE RIDGE OVER THE
AREA. ANOTHER SYSTEM MAY MOVE IN FOR LATER THURSDAY NIGHT BUT WITH
MODELS DISAGREEMENT WE KEPT THINGS MAINLY DRY FOR NOW.

EAST OF THE DIVIDE...A TRICKY FORECAST HERE AS WELL. FIRST FOR THE
POTENTIAL FOR SNOW. AGAIN...THE MODELS ARE SPLIT. THE NAM IS THE
DRIEST AGAIN WITH THE GFS AND EUROPEAN A BIT WETTER. CONTINUITY FOR
THE BIG HORN RANGE LOOKS PRETTY GOOD AT THIS TIME WITH FAVORABLE
NORTHWEST FLOW BRINGING THE BEST ACCUMULATIONS TO THE WESTERN
SLOPES. SOME AREAS COULD REACH WARNING CRITERIA BUT WE STILL THINK
THIS SHOULD BE FAIRLY ISOLATED. THE BIG QUESTION IS FOR THE
BASINS...ESPECIALLY THE BIG HORN BASIN. SOME OF THE MODELS DO BRING
A DECENT AMOUNT OF QPF IN THAT AREA AS A BACKDOOR COLD FRONT GETS
OVERRUN BY THE MOISTURE COMING IN FROM THE WEST. IT LOOKS LIKE A
FAIRLY GOOD CHANCE OF PRECIPITATION FOR SOME AREAS...ESPECIALLY THE
EASTERN BIG HORN BASIN AND JOHNSON COUNTY SO WE BUMPED THE POPS UP
TO LIKELY. THE NEXT QUESTION IS HOW MUCH SNOW WILL FALL.
TEMPERATURES ARE FAIRLY BORDERLINE HERE AS WELL. AGAIN...AS A
COMPROMISE WE RAISED AMOUNTS OF QPF AND SNOW. WE DID KEEP THE
AMOUNTS UNDER ADVISORY LEVELS HOWEVER. WE GUT FEELING IS THAT THE
BEST CHANCE OF ACCUMULATING SNOW WOULD BE TONIGHT SO THIS COULD BE
RE-EVALUATED BY THE DAY SHIFT. LIKE WEST OF THE DIVIDE...COVERAGE
WILL DECREASE FROM THE SOUTH TO NORTH AS THE BEST SUPPORT MOVES AWAY
AND RIDGING BUILDS ACROSS THE AREA. AT THIS TIME...THE PERIOD FROM
WEDNESDAY NIGHT THROUGH THURSDAY NIGHT LOOKS MAINLY DRY.

NOW FOR ACT TWO...THE HIGH WIND. AS EXPECTED...THE MOST IMPORTANT
CROSSROADS IN AMERICA...CLARK...HAS ALREADY REPORTED WINDS OVER 60
MPH. HOWEVER...STRONG WINDS HAVE MOVED OFF THAT WIND PRONE AREA WITH
A GUST TO 56 MPH REPORTED AT THE CODY AIRPORT. FOR NOW...WE WILL
CONTINUE THE HIGH WIND WARNING. HOWEVER...WINDS WILL BE RATHER GUSTY
ACROSS MUCH OF THE AREA TODAY. AT THIS TIME...NO FURTHER HIGH WIND
WARNINGS ARE ANTICIPATED. THE MAV MOS AT ROCK SPRINGS DOES HAVE HIGH
WIND CRITERIA...BUT THIS IS USUALLY OVERDONE.  FOR NOW...WE WILL
CONTINUE THE SPECIAL WEATHER STATEMENT BUT THIS WILL HAVE TO BE
WATCHED...ESPECIALLY FOR ZONE 30. THE WIND SHOULD DECREASE TONIGHT
AS THE SUPPORTING JET STREAK MOVES AWAY AND THE PRESSURE GRADIENT
LESSENS. GUSTY BREEZES WILL CONTINUE FROM TUESDAY NIGHT THROUGH
WEDNESDAY NIGHT ALTHOUGH NOT AS STRONG AS TODAY.

LONG TERM...FRIDAY THROUGH TUESDAY

MEDIUM RANGE STARTS OUT WITH A FLAT RIDGE AXIS JUST TO OUR WEST
WHILE A COLD TROUGH MOVES SSEWD INTO SRN B.C. MILD AIR AHEAD OF THIS
WILL LIKELY LEAD TO RATHER MILD LATE NOVEMBER DAY ACROSS MUCH OF THE
AREA...ESPECIALLY EAST OF THE DIVIDE WHERE THERE WILL BE 50S IN MANY
AREAS. WINDS MAY ALSO BE A SIGNIFICANT FACTOR...ESPECIALLY FOR THE
MOUNTAINS...WIND CORRIDOR AND CODY FOOTHILLS AS STRONG WLY FLOW
COMBINES WITH A STRENGTHENING SFC PRESSURE GRADIENT AS A STRONG LOW
FORMS OVER MONTANA. THE COLD TROUGH TO THE NW HAS ITS ORIGINS AROUND
THE NORTH POLE AND WILL LIKELY CONTAIN QUITE A BIT OF COLD AIR.
RIGHT NOW THE MODELS ARE KEEPING THE MAJORITY OF THE ENERGY TO THE
NORTH BUT DO HAVE AT LEAST A SHALLOW...COLD AIRMASS TRYING TO SETTLE
IN EAST OF THE DIVIDE OVER THE WEEKEND. THE EURO IS COLDER
AGAIN...FASTER AND FURTHER SOUTH WITH THE COLD AIR PUSH BRINGING THE
COLD AIR INTO THE NORTH QUICKLY SATURDAY MORNING AND TO ALL AREAS
EAST OF THE DIVIDE BY SATURDAY AFTERNOON. THE GFS IS NOT AS COLD
AND HINTS AT KEEPING THE FRONT JUST NORTH OF THE CENTRAL ZONES.
WILL KEEP THE CENTRAL ZONES ABOVE NORMAL THROUGH SATURDAY WITH
SOME COOLING ON SUNDAY. THE FAR NRN ZONES MAY BE QUITE CHILLY WITH
THIS SYSTEM BUT WITH LIMITED FORCING AND THUS NOT MUCH SNOW FOR
THE LOWER ELEVATIONS. IN THE WEST AND NRN MOUNTAINS...A MOIST WLY
FLOW WILL LIKELY LEAD TO SOME LIGHT TO MODERATE SNOW AT TIMES
THROUGH THE WEEKEND WITH POTENTIAL FOR SOME OVERRUNNING EAST OF
THE DIVIDE IF THE LOW LEVEL COLD AIR BECOMES ENTRENCHED. EARLY
NEXT WEEK...WE GET A NEW SYSTEM MOVING ONTO THE WEST COAST THAT IS
FORECAST TO MOVE ACROSS US ON TUESDAY. THIS SYSTEM IS THE ONE NEAR
KAMCHATKA RIGHT NOW AND EVENTUALLY THERE IS A SPLIT WITH THE SRN
PIECE COMING ON THE CA/OR COAST WITH THE NRN PIECE PROVIDING SOME
COLDER AIR AS IT MOVES INLAND. LOT OF DETAIL ISSUES THIS FOR OUT
SO CONFIDENCE ON TIMING AND PCPN POTENTIAL IS LOW RIGHT NOW. WILL
KEEP MOST OF THE SNOW POTENTIAL CONFINED TO THE WEST A LITTLE IN
SOUTH FOR NOW AS THIS SYSTEM MOVES THROUGH. LATER SHIFTS CAN WATCH
THIS EVOLUTION.

AVIATION.../18Z ISSUANCE/

EAST OF THE DIVIDE...KCOD/KCPR/KLND/KRIW/KWRL ROUTES

OVER THE MOUNTAINS AND NORTH IFR TO MVFR CEILINGS AND VISIBILITIES
REDUCED IN SNOW AND BLOWING SNOW WILL BE WIDESPREAD THROUGH 12Z
WEDNESDAY...THEN DECREASE.  TO THE SOUTH VFR CONDITIONS ARE
EXPECTED IN THE WIND RIVER BASIN AND SOUTHERN NATRONA COUNTIES
UNTIL 00Z THEN IFR/MVFR CONDITIONS WILL SPREAD SOUTH BEHIND THE
COLD FRONT UP TO THE CONTINENTAL DIVIDE.  STRONG WEST WINDS 30 TO
40 KTS WITH GUSTS TO 60KTS ARE EXPECTED ALONG AND NEAR THE EAST
SLOPES OF THE MOUNTAINS UNTIL 06Z. WINDS WILL SHIFT TO THE NORTH
AS A COLD FRONT MOVES SOUTH FROM MONTANA.

WEST OF THE DIVIDE...KBPI/KJAC/KPNA/KRKS ROUTES

IFR TO LOCAL MVFR CEILINGS AND VISIBILITIES REDUCED IN SNOW AND
BLOWING SNOW WILL BE ACROSS THE EAST THROUGH 12Z WEDNESDAY WITH
GRADUAL IMPROVEMENT FROM 12Z TO 18Z.  STRONG WEST WINDS 30 TO 40
KTS WITH GUSTS TO 60KTS ARE EXPECTED ALONG AND NEAR THE EAST
SLOPES OF THE MOUNTAINS UNTIL 06Z.

FIRE WEATHER...

A MOIST NORTHWEST FLOW WILL RESULT IN SNOW AND STRONG WIND FOR MOST
MOUNTAIN RANGES THROUGH TODAY. THESE CONDITIONS WILL KEEP FIRE
WEATHER IMPACTS TO A MINIMUM. THE SAME LOW IMPACT APPLIES TO THE
LOWER ELEVATIONS OF WEST CENTRAL WYOMING...ALTHOUGH GUSTY SURFACE
WIND REMAINS POSSIBLE THROUGH THIS AFTERNOON. SNOW SHOWERS WILL DRIFT
OFF THE HIGHER TERRAIN OR FOOTHILLS DURING THIS TIME...ESPECIALLY
ACROSS THE BIG HORN BASIN AND JOHNSON COUNTY.  HIGH PRESSURE AND
DRIER/WARMER WEATHER CAN BE EXPECTED FOR FIRE DISTRICTS ON
WEDNESDAY AND THURSDAY.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
HIGH WIND WARNING UNTIL 11 PM MST THIS EVENING WYZ016-027>030.

WINTER WEATHER ADVISORY UNTIL 5 AM MST WEDNESDAY
WYZ003-005>007-010-011-025.

WINTER STORM WARNING UNTIL 5 AM MST WEDNESDAY
WYZ001-002-004-008-009-012>015-024.

WINTER STORM WARNING UNTIL MIDNIGHT MST TONIGHT WYZ023.

&&

$$

SHORT TERM...HATTINGS
LONG TERM...SKRBAC
AVIATION...C.BAKER
FIRE WEATHER...HATTINGS






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